IEEE (Institute of Electrical and Electronics Engineers) 802.16 standards provide techniques and protocols to support a broadband wireless access. Standardization proceeded starting from 1999 and IEEE 802.16-2001 was approved in 2001. It is based on a single carrier physical layer called ‘WirelessMAN-SC’. Later, besides the ‘WirelessMAN-SC’, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ were added to the physical layer in IEEE 802.16a standards approved in 2003. After the IEEE 802.16a standards were completed, revised IEEE 802.16-2004 standards were approved in 2004. IEEE 802.16-2004/Cor1 (referred to as ‘IEEE 802.16e’ hereinafter) was finalized in the form of ‘corrigendum’ in 2005 in order to resolve and correct bugs and errors of the IEEE 802.16-2004 standards.
An error correction scheme for securing the reliability of communications includes a forward error correction (FEC) scheme and an automatic repeat request (ARQ) scheme. In the FEC scheme, an extra error correction code is added to information bits to correct an error in a receiving end. In the ARQ scheme, an error is corrected through data retransmission, and the ARQ scheme includes a stop and wait (SAW) scheme, a go-back-N (GBN) scheme, a selective repeat (SR) scheme, or the like. The SAW scheme is a scheme in which it is checked whether or not a transmitted frame has been properly received and then a next frame is transmitted. The GBN scheme is a scheme in which the N number of successive frames are transmitted, and if the transmission is not successfully made, all the frames, which were transmitted following a frame with an error, are retransmitted. The SR scheme is a scheme for selectively retransmitting only frame with an error.
The FEC scheme is advantageous in that a time delay is small and there is no need to transmit or receive information between a transmitting end and the receiving end, but it has shortcomings that a system efficiency deteriorates in a good channel environment. The ARQ scheme ensures a high transmission reliability but is disadvantageous in that time delay occurs and the system efficiency deteriorates in a bad channel environment. A hybrid automatic repeat request (HARQ) scheme combining the FEC and the ARQ has been proposed to resolve such drawbacks. According to the HARQ scheme, it is checked whether data received by a physical layer has an error that cannot be decoded, and if the data has an error, the data is requested to be retransmitted to thereby enhance performance.
A mode of the HARQ may be divided into a chase combining (CC) mode and an incremental redundancy (IR) mode. The CC mode is to obtain a signal-to-noise ratio (SNR) gain by combining an error-detected data with retransmitted data, rather than discarding the error-detected data. The IR mode is to incrementally transmit additional redundant information of retransmitted data to thereby reduce a burden of retransmission and obtain a coding gain.
If no error is detected from received data, a receiver transmits an acknowledgement (ACK) signal as a response signal to inform a transmitter about the successful reception. If an error is detected from received data, the receiver transmits a negative-acknowledgement (NACK) signal as a response signal to inform the transmitter about the error detection. Upon receiving the NACK signal, the transmitter may retransmit data.
The receiver of the HARQ scheme basically attempts an error correction on the received data and determines whether to retransmit data by using an error detection code. As the error detection code, a cyclic redundancy check (CRC) may be used. When an error of reception data is detected through the CRC detection process, the receiver transmits the NACK signal to the transmitter. Upon receiving the NACK signal, the transmitter transmits proper retransmission data according to the HARQ mode (CC mode or IR mode). Upon receiving the retransmission data, the receiver combines the previous data and the retransmission data and decodes the same, thereby improving reception performance.
The retransmission scheme of the HARQ may be classified into a synchronous scheme and an asynchronous scheme. In the synchronous HARQ, data is retransmitted at a point of time both the transmitter and the receiver knows about, and signaling required for transmission of data such as HARQ processor number or the like can be reduced. In the asynchronous HARQ, resources are allocated at an arbitrary time for retransmission. Because signaling is required for data transmission, overhead is generated.
The HARQ may be classified into an adaptive HARQ and a non-adaptive HARQ according to transmission attributes such as a resource allocation, modulation scheme, size of a transport block, or the like. The adaptive HARQ is a scheme in which the transmission attributes used for retransmission are compared with a first transmission, and entirely or partially changed and transmitted according to a change in a channel state. The non-adaptive HARQ is a scheme in which the transmission attributes used for a first transmission are continuously used regardless of a change in a channel state.
When data is retransmitted in the IR mode, additional redundant information should be incrementally transmitted, but in case of the adaptive HARQ scheme in which transmission attributes are changed, the modulation scheme, the size of a transport block, or the like, vary, so the retransmission data cannot be incrementally transmitted and the coding gain of the IR mode cannot be obtained.
Thus, a method for obtaining the gain of the IR mode although the amount of retransmission data is arbitrarily changed due to a change in the modulation scheme, resource allocation, or the like, in the adaptive HARQ scheme is required.